In a conventional display controller, a display area setting element is limited to only to one setting method. For example, in case a flat display panel having a size of 640 dots (horizontal direction) 400 dots (vertical direction) is used, the display area (display size) of controller is set to 640.times.400 dots, while in case a flat display of 640 dots (horizontal direction) by 200 dots (vertical direction) in size is used, the display area of the controller is changed to 640.times.200 dots. Therefore, in case a plurality of display areas are used in one system; namely in case two kinds of display areas such as the in sizes of 640.times.400 and 640.times.200 dots are provided, two different kinds of flat displays are prepared and the display hardware corresponding to the desired display mode is selected.
Since a conventional display controller employs only one kind of display area setting element, if only one display area is prepared or desired in one system, no problem occurs but in case many kinds of display areas are used depending on the software in one system, the display hardware used must be selected to correspond to the software output. Namely, many different types of software cannot be displayed on a single display. For example, in the case of a personal computer IBM-PC type as manufactured by IBM, a plurality of display areas such as 640.times.350 dots or 640.times.200 dots, etc. are prepared electronically available for use in the system. Since the display area is selected by the software, if the display hardware is fixed to say, 640.times.350 dots, software which supports only the display of 640.times.200 dots can no longer be used by the system.
It is therefore an object of the present invention to solve a problem that exists when a plurality of display areas cannot be displayed on only one display unit by a mode setting method of display controller in order to widen the application field even if the display areas change.